The present invention relates to a radio pager of the type alerting the user to a call by vibration and, more particularly, to a motor applicable to such a radio pager for providing a vibrating action.
A radio pager is generally provided with alerting means in the form of, for example, a speaker for producing an alert tone or a motor for generating vibration. The motor for providing a vibrating action usually has a casing and a rotor assembly disposed in the casing. The rotor assembly is made up of a rotor partly removed and rotatably mounted on a shaft, and coils mounted on the rotor. The center of gravity of the partly removed motor is eccentric to the shaft. As a result, the center of gravity of the motor changes during the course of rotation to provide a vibrating action. However, removing part of the rotor correspondingly reduces the overall mass of the rotor assembly and, therefore, the change of the center of gravity relative to the shaft during rotation, thereby preventing the motor from generating a sufficient degree of vibration. Further, the removed part of the rotor prevents the coil from having a great number of turns. This not only limits the torque and, therefore, vibration available with the motor but also aggravates current consumption. U.S. Pat. No. 4,864,276 to Tribbey et al discloses a rotor assembly capable of making the center of gravity of the rotor eccentric to the shaft despite that the rotor is implemented as a disk. This rotor assembly is characterized in that one of a plurality of coils mounted on the rotor is smaller in size and weight than the others. However, even with this kind of scheme, it is impossible to achieve a satisfactory degree of vibration.